Forming refractory masses and composition of matter for use in forming such refractory masses

ABSTRACT

In processes of forming a refractory mass on a surface, a mixture of refractory particles and oxidizable particles which react exothermically with oxygen to generate sufficient heat to soften or melt at least the surfaces of the refractory particles are sprayed against that surface to bring about formation of said refractory mass. To promote the reliable and consistent formation of a durable refractory mass, the granulometry of the particles which are sprayed in the mixture is such that the 80% and 20% grain sizes of the refractory particles (that is, the screen mesh sizes G 80  and G 20  through which respectively 80% and 20% by weight of the particles will pass) have a mean greater than the mean of the 80% and 20% grain sizes of the oxidizable particles and the size range spread factor f(G) of the refractory particles is at least 1.2, where ##EQU1## In a composition of matter for spraying against a surface to form a refractory mass, which consists of such a mixture, the granulometric requirements are the same, and the exothermically oxidizable particles are present in an amount between 5% and 30% by weight of the mixture, and the granulometry of said particles is such that the mean of the 80% and 20% grain sizes of the refractory particles is greater than the mean of the 80% and 20% grain sizes of the oxidizable particles and that the size range spread factor (as herein defined) of the refractory particles is at least 1.2.

This is a division of application Ser. No. 06/803,782 filed Dec. 2,1985, now U.S. Pat. No. 4,792,468.

This invention relates to a process of forming a refractory mass on asurface, which process comprises spraying against that surface a mixtureof refractory particles and oxidisable particles which reactexothermically with oxygen to generate sufficient heat to soften or meltat least the surfaces of the refractory particles and so bring aboutformation of said refractory mass.

This invention also relates to a composition of matter for sprayingagainst a surface to form a refractory mass, such composition being amixture comprising refractory particles, together with particles ofexothermically oxidisable material.

Processes of the kind referred to are particularly suitable for the hotrepair of furnaces and other refractory devices. They are also useful inthe formation of refractory components, for example for the surfacing ofrefractory metals or other refractory substrates, and in particular forthe formation of refractory linings on parts which are especiallysusceptible to erosion. In the case of furnace repair, such processescan be, and indeed preferably are, operated substantially at the workingtemperature of the furnace. In addition, in some cases, for example therepair of a glass melting furnace superstructure, repair can be effectedwhile the furnace is still operating.

It is important that the refractory mass formed should be of a highquality so that it will have a long useful working life. It has beenfound that the ability of such a mass to resist erosion and otherstresses, in particular thermal stresses, to which it is likely to besubjected during its working life is dependent not only on itscomposition, but also on its structure, and that the structure of therefractory mass is strongly influenced by the way the mass forms fromthe sprayed material.

It is an object of the present invention to provide a new process offorming a refractory mass on a surface, which process provides certainadvantages as will hereinafter be adverted to.

According to the present invention, there is provided a process offorming a refractory mass on a surface, which process comprises sprayingagainst that surface a mixture of refractory particles and oxidisableparticles which react exothermically with oxygen to generate sufficientheat to soften or melt at least the surfaces of the refractory particlesand so bring about formation of said refractory mass, characterised inthat the granulometry of the particles which are sprayed in the mixtureis such that the mean of the 80% and 20% grain sizes of the refractoryparticles is greater than the mean of the 80% and 20% grain sizes of theoxidisable particles and that the size range spread factory (as hereindefined) of the refractory particles is at least 1.2.

The expression "% grain size" used herein in relation to particles ofmaterial denotes that that % proportion by weight of the particles willpass a screen having a mesh of that size, and references to the mean oftwo grain sizes are references to half the sum of those grain sizes.

The expression "size range spread factor" [f(G)] is used herein inrespect of a given species of particles to denote the factor: ##EQU2##where G₈₀ denotes the 80% grain size of the particles of that species,and G₂₀ denotes the 20% grain size of the particles of that species.

In general, samples of particles of a given material have a size rangedistribution which follows a bell curve, and when the cumulativedistribution, that is the proportion by weight which will pass a screenhaving a mesh of a given size, is plotted on a linear scale against thescreen mesh size plotted on a logarithmic scale, the result is a sigmoidcurve which is generally straight between the points which correspond tothe 80% and 20% grain sizes of the particles under test.

It has been found that the observance of the specified conditions inrespect of the granulometry of the sprayed particles promotes thereliability and consistency with which highly durable refractorydeposits can be formed under given process conditions. It is extremelysurprising that the granulometry of the sprayed particles should havesuch an effect on the quality of the refractory product, particularlysince it has been found that this advantage accrues even when theprocess is performed under conditions such that the refractory particlessprayed become completely melted. The reliable and consistent formationof a durable refractory mass by the adoption of a process according tothis invention is attributed to a tendency for that refractory productto be comparatively less porous and comparatively free from cracks withrespect to a refractory product formed by a process in which the saidparticle granulometry conditions are not observed but which is otherwisesimilar. The high size range spread factor presumably contributes tothis result, but it has been found that reliance on that factor alone isnot sufficient to give good results. Notwithstanding the wide size rangespread of the refractory particles, it has been found that theoxidisable particles must be of a lower mean size (as hereinbeforedefined), or the advantage referred to pertaining to the quality of therefractory masses formed by the process will not be achieved. It will beappreciated that for any given proportion of oxidisable particles of agiven composition present in the mixture, the number of such particlespresent will vary inversely with the cube of their mean size. It isimportant to have a large number of such particles present to allowdirect radiant heating of substantially all the refractory particlesduring spraying.

In preferred embodiments of the invention, the mean of the 80% and 20%grain sizes of the refractory particles is not greater than 2.5 mm. Theadoption of this condition favours the smooth progression of the processas regards the particle supply to and flow through the lance. In orderfurther to promote this smooth operation it is particularly preferredthat the 90% grain size of the refractory particles is not greater than4 mm.

Advantageously, the mean of the 80% and 20% grain sizes of therefractory particles is not greater than 1 mm, and the 90% grain size ofthe refractory particles is not greater than 2 mm. Not only does theadoption of this feature yet further promote smooth operation, but also,it has been found that if larger particles are used, they occasionallytend to rebound from a surface against which they are sprayed soreducing the amount of material deposited to form a refractory mass.This is especially so when the particles are sprayed against an overheadsurface. By keeping the mean of the 80% and 20% grain sizes of therefractory particles and their 90% grain size down to those values, thistendency is much reduced.

Advantageously, the mean of the 80% and 20% grain sizes of therefractory particles is at least 50 μm. The adoption of this preferredfeature helps to avoid smothering of the oxidation reactions which takeplace during spraying of the mixture, by refractory particles which aretoo small.

Preferably, the size range spread factory (as herein defined) of therefractory particles is at least 1.3. This further promotes a reductionin porosity of a refractory product formed on spraying of the mixture.

Advantageously, the size range spread factor (as herein defined) of therefractory particles is not greater than 1.9. This condition isbeneficial for reducing segregation of different sizes of refractoryparticles by settling during storage or handling, for example duringfeed to the lance.

In preferred embodiments of the invention, the size range spread factor(as herein defined) of the oxidisable particles is not greater than 1.4.As contrasted with the size range spread factor of the refractorycomponent of the mixture, a high size range spread factor for theoxidisable particles is not to be sought after as it militates againstthe uniformity of the oxidation reactions which is desirable for theformation of a high quality refractory mass. The size range spreadfactor (as herein defined) of the oxidisable particles may for examplebe 1.3 or less.

Advantageously, the mean of the 80% and 20% grain sizes of theoxidisable particles is not greater than 50 μm. Particles of such grainsizes are easily oxidised, so promoting rapid evolution of heat duringspraying of the mixture.

Such rapid oxidation and evolution of heat during spraying is furtherpromoted when, as is preferred, the 90% grain size of the oxidisableparticles is not greater than 50 μm.

In order further to promote rapid oxidation, it is preferred that themean of the 80% and 20% grain sizes of the oxidisable particles is notgreater than 15 μm. The adoption of this preferred feature enables theoxidation reactions to proceed sufficiently rapidly to ensuresubstantially complete combustion of the oxidisable particles withoutadding unnecessarily to the cost of the starting materials.

Incombustible particles of various compositions can be used in thepresent invention, depending of course on the required composition ofthe refractory mass to be deposited on spraying of the mixture. Ingeneral, for compatibility between such a refractory deposit and asurface of a refractory substrate on which it is formed and to which itis to remain adherent, it is desirable that the deposit should includematerial having a similar chemical composition to material included inthe substrate. Problems which can arise if this general guideline is notfollowed may be due to chemical incompatibility between the deposit andthe substrate, or to a wide difference between their coefficients ofthermal expansion which could lead to excessive thermal stress at theirboundary and flaking off of the deposited refractory mass. The mostuseful materials for forming said refractory particles comprise one ormore of sillimanite, mullite, zircon, SiO₂, ZrO₂, Al₂ O₃, MgO.

Preferably, at least some of the refractory material has previously beenfired to a temperature in excess of 0.7 times its melting pointexpressed in kelvins. Such a heat treatment has a beneficial effect onvarious refractory materials for promoting the formation of a highquality refractory deposit. In the case of some materials, such asmagnesia, such a heat treatment drives off any molecular water bound inthat material. In the case of other materials, for example silica, sucha heat treatment favourably alters the crystallographic structure forthe purpose in view.

When the refractory material comprises particles of silica, it has beenfound that the mineralogical form of the silica has an important effecton the form of the silica incorporated in a refractory mass formed byspraying the mixture, notwithstanding that the silica may have beencompletely melted during such spraying. Preferably, at least 90% byweight of any silica present in said refractory material of said mixtureis in the form of tridymite and/or cristobalite, as this gives the bestresults.

Indeed, it has been found in general that the crystallographic structureof the refractory product formed by a process according to thisinvention is strongly influenced, if not even determined, by the formand size of the material sprayed. It is assumed that even if therefractory particles sprayed do become completely melted, somecrystallites remain in the fluid state to influence the way in whichrecrystallisation takes place on subsequent solidification.

Advantageously, said oxidisable particles comprise particles of one ormore of silicon, aluminium, magnesium and zirconium. Particles of suchmaterials can be oxidised rapidly with a high accompanying evolution ofheat and themselves form refractory oxides, and are thus very suitablefor use in the present invention.

For economic reasons, it is preferred that said oxidisable particles arepresent in an amount not exceeding 20% by weight of said mixture. Thereis also a technical reason for that limit, in that if greaterproportions of oxidisable material are used, the working surface may beliable to overheat.

The present invention also provides a composition of matter for sprayingagainst a surface to form a refractory mass, such composition being amixture comprising refractory particles, together with particles ofexothermically oxidisable material, characterised in that theexothermically oxidisable particles are present in an amount between 5%and 30% by weight of said mixture and the granulometry of said particlesis such that the mean of the 80% and 20% grain sizes of the refractoryparticles is greater than the mean of the 80% and 20% grain sizes of theoxidisable particles and that the size range spread factor (as hereindefined) of the refractory particles is at least 1.2.

Such a composition contributes to the facility with which durablerefractory masses can be formed by causing combustion of the oxidisableparticles during spraying, and the latitude allowed for the grain sizerange spread of the refractory particles has a favourable effect on theproduction costs of the composition. The said mixture can be formedusing refractory particles which are readily obtainable by anappropriate selection of sizing operations.

Advantageously, the mean of the 80% and 20% grain sizes of therefractory particles is not greater than 2.5 mm. The adoption of thiscondition is favourable for smooth feed of the particles to and througha lance which is to be used for spraying the particles. In order furtherto promote this smooth feed it is particularly preferred that the 90%grain size of the refractory particles is not greater than 4 mm.

Preferably, the mean of the 80% and 20% grain sizes of the refractoryparticles is not greater than 1 mm, and the 90% grain size of therefractory particles is not greater than 2 mm. Not only does theadoption of this feature yet further promote smooth feed of theparticles, but also, it has been found that if larger particles areused, they occasionally tend to rebound when they are sprayed against asurface, so reducing the amount of material which would be deposited toform a refractory mass. This is especially so when the particles are tobe sprayed against an overhead surface. By keeping the mean of the 80%and 20% grain sizes of the refractory particles and their 90% grain sizedown to those values, this tendency is much reduced.

Advantageously, the mean of the 80% and 20% grain sizes of therefractory particles is at least 50 μm. The adoption of this preferredfeature helps to avoid smothering of the oxidation reactions which takeplace when the mixture is sprayed, by refractory particles which are toosmall.

Preferably, the size range spread factor (as herein defined) of therefractory particles is at least 1.3. This further promotes a reductionin porosity of a refractory formed when the mixture is sprayed.

Advantageously, the size range spread factor (as herein defined) of therefractory particles is not greater than 1.9. This limits the size rangespread of those particles so that a given sample will have a relativelylow proportion of particles which are either comparatively very small orvery large. The adoption of this feature gives a reduced tendency forthe particles to segregate by settling during transport from place toplace or even while they are contained in a hopper of a sprayingmachine.

In preferred embodiments of the invention, the size range spread factor(as herein defined) of the oxidisable particles is not greater than 1.4.As contrasted with the size range spread factor of the refractorycomponent of the mixture, a high size range spread factor for theoxidisable particles is not to be sought after as it militates againstthe uniformity of the oxidation reactions which is desirable for theformation of a high quality refractory mass when the mixture is sprayed.The size range spread factor (as herein defined) of the oxidisableparticles may for example be 1.3 or less.

Advantageously, the mean of the 80% and 20% grain sizes of theoxidisable particles is not greater than 50 μm. Particles of such grainsizes are easily oxidised, so promoting rapid evolution of heat when themixture is sprayed.

Such rapid oxidation and evolution of heat when the mixture is sprayedis further promoted when, as is preferred, the 90% grain size of theoxidisable particles is not greater than 50 μm.

In order further to promote rapid oxidation, it is preferred that themean of the 80% and 20% grain sizes of the oxidisable particles is notgreater than 15 μm. The adoption of this preferred feature enables theoxidation reactions which take place when the mixture is sprayed toproceed sufficiently rapidly to ensure substantially complete combustionof the oxidisable particles without adding unnecessarily to the cost ofthe starting materials.

Incombustible particles of various compositions can be used in thepresent invention, depending of course on the required composition ofthe refractory mass to be deposited on spraying of the mixture. Ingeneral, for compatibility between such a refractory deposit and asurface of a refractory substrate on which it is to be deposited and towhich it is to remain adherent, it is desirable that the deposit shouldinclude material having a similar composition to material included inthe substrate. Problems which can arise if this general guideline is notfollowed may be due to chemical incompatibility between the deposit andthe substrate, or to a wide difference between their coefficients ofthermal expansion which could lead to excessive thermal stress at theirboundary and flaking off of the deposited refractory mass. The mostuseful materials for forming said refractory particles comprise one ormore of sillimanite, mullite, zircon, SiO₂, ZrO₂, Al₂ O₃, MgO.

Preferably, at least some of the refractory material has previously beenfired to a temperature in excess of 0.7 times its melting pointexpressed in °Kelvin. Such a heat treatment has a beneficial effect onvarious refractory materials for promoting the formation of a highquality refractory deposit when the mixture is sprayed. In the case ofsome materials, such as magnesia, such a heat treatment drives off anymolecular water bound in that material. In the case of other materials,for example silica, such a heat treatment favourably alters thecrystallographic structure for the purpose in view. The refractoryparticles can readily be obtained by an appropriate selection of sizingoperations.

When the refractory material comprises particles of silica, it has beenfound that the mineralogical form of the silica has an important effecton the form of the silica incorporated in a refractory mass formed byspraying the mixture, notwithstanding that the silica may have beencompletely melted during such spraying. For the best results, it hasbeen found that at least 90% by weight of any silica present in saidrefractory material of said mixture should be in the form of tridymiteand/or cristobalite, as is preferred.

Indeed, it has been found in general that the crystallographic structureof the refractory product formed by a process according to thisinvention is strongly influenced, if not even determined, by the formand size of the material sprayed. It is assumed that even if therefractory particles sprayed do become completely melted, somecrystallites remain in the fluid state to influence the way in whichrecrystallisation takes place on subsequent solidification.

Advantageously, said oxidisable particles comprise particles of one ormore of silicon, aluminium, magnesium and zirconium. Particles of suchmaterials can be oxidised rapidly with a high accompanying evolution ofheat and themselves form refractory oxides, and are thus very suitablefor use in the present invention.

For economic reasons, it is preferred that said oxidisable particles arepresent in an amount not exceeding 20% by weight of said mixture.

The following are some examples of processes and compositions of matteraccording to the invention.

In Examples 1 and 2, reference will also be made to the accompanyingdrawing which shows a graph of the cumulative distribution of variousspecies of particles used, that is the proportion by weight which willpass a screen having a mesh of a given size, in which the cumulativeproportion is plotted on a linear scale against the screen mesh sizeplotted on a logarithmic scale.

EXAMPLE 1

A mixture of particles was prepared comprising by weight 20% silicon and80% silica. The silica was obtained by crushing bricks made from quartzsand which had previously been fired at a temperature of at least 1400°C. Due to the firing, two parts by weight of the silica were in the formof tridymite and three parts by weight were in the form of cristobalite.

Cumulative size range distribution graphs of the silicon and silica usedare shown in the accompanying drawing.

The granulometry of the various particles is also given in the followingtable in which G₂₀, G₈₀ and G₉₀ respectively are the 20%, 80% and 90%grain sizes of the particles and f(G) is their size range spread factoras herein defined.

    ______________________________________                                        Material  G.sub.20 μm                                                                         G.sub.80 μm                                                                           G.sub.90 μm                                                                       f(G)                                     ______________________________________                                        Si         3        14          19.5 1.29                                     SiO.sub.2 170      1020       1450   1.43                                     ______________________________________                                    

The mixture of particles was projected at a rate of 1 kg/min in a streamof oxygen delivered at 200 L/min using apparatus as described in BritishPatent Specification No 1,330,895 to form a uniform adherent refractorycoating on a silica furnace wall which was at a temperature of 1200° C.to 1250° C. The use of the mixture led to the formation of substantiallycrack-free refractory coatings which adhered very well to the workingsurface. In addition, it was found that the boundary between thedeposited coating and the original wall was substantially crack-freeeven when the coating was deposited to a thickness of 5 cm or more. Thepresence of boundary cracks is a particular problem when depositingsilica coatings on silica walls. By way of comparison it has been foundthat when a mixture which did not have a granulometry in accordance withthis invention was sprayed by a similar method, even when the coatingthickness was as low as 1 cm, cracks were present both in the coatinglayer itself and at its boundary with the working surface of the wall.

The particulate refractory material substituted into the startingmixture for the purposes of this comparison was natural quartz sandhaving the following granulometry.

    ______________________________________                                        Material   G.sub.20 μm                                                                         G.sub.80 μm                                                                           G.sub.90 μm                                                                       f(G)                                    ______________________________________                                        Quartz sand                                                                              55       190        250    1.1                                     ______________________________________                                    

EXAMPLE 2

A mixture of particles was prepared comprising by weight 8% silicon, 4%aluminium and 88% magnesia. The magnesia used was natural magnesia whichhad been fired at 1900° C. in order to dehydrate it.

The silicon used had the granulometry specified in Example 1. Cumulativesize range distribution graphs of the aluminium and magnesia used arealso shown in the accompanying drawing.

The granulometry of the various particles is also given in the followingtable.

    ______________________________________                                        Material  G.sub.20 μm                                                                         G.sub.80 μm                                                                           G.sub.90 μm                                                                       f(G)                                     ______________________________________                                        Si        3        14         19.5   1.29                                     Al        4.6      15         19.5   1.06                                     Si + Al   3.5        14.4     19.5   1.22                                     MgO       90       1110       1500   1.7                                      ______________________________________                                    

The mixture of particles was projected using the same apparatus as inExample 1, to form a uniform adherent refractory coating on a furnacewall which built up of basic refractory blocks mainly consisting ofmagnesia and which was at a temperature in excess of 1000° C. The use ofthe mixture led to the formation of low-porosity refractory coatingswhich adhered very well to the working surface.

EXAMPLE 3

A mixture of particles was prepared comprising by weight 6% silicon, 6%aluminium and 88% zircon/zirconia and alumina. The refractory particleswere obtained by crushing used or broken electrocast refractory blocksof the type available under the Trade Mark "Corhart Zac". Theapproximate composition by weight of those blocks was: Al₂ O₃ 65-75%;ZrO₂ 15-20%; SiO₂ 8-12%.

The silicon, aluminium and refractory particles had the followinggranulometry:

    ______________________________________                                        Material   G.sub.20 μm                                                                         G.sub.80 μm                                                                           G.sub.90 μm                                                                       f(G)                                    ______________________________________                                        Si         3        14         19.5   1.29                                    Al         4.6      15         19.5   1.06                                    Si + Al    3.6      14.8       19.5   1.21                                    Refractory 52.5     248        ˜330                                                                           1.3                                     ______________________________________                                    

This starting mixture was projected using the same apparatus as in theprevious Examples onto an aluminous refractory wall to depositsubstantially crack-free, low porosity coatings.

We claim:
 1. A composition of matter for spraying against a surface toform a refractory mass, such composition being a mixture comprisingrefractory particles, together with particles of exothermicallyoxidisable material, wherein the exothermically oxidisable particles arepresent in an amount between 5% and 30% by weight of said mixture andthe granulometry of said particles is such that the mean of the 80% and20% grain sizes of the refractory particles is greater than the mean ofthe 80% and 20% grain sizes of the oxidisable particles and that thesize range spread factor, f(G), of the refractory particles is at least1.2 wherein the % grain size of the particles of material denotes that %proportion by weight of the particles which will pass a screen having amesh of that size, and the mean of two % grain sizes refers to half ofthe sum of those grain sizes, and ##EQU3## where G₈₀ denotes the 80%grain size of particles of that species and G₂₀ denotes the 20% grainsize of particles of that species.
 2. A composition of matter accordingto claim 1, wherein the mean of the 80% and 20% grain sizes of therefractory particles is not greater than 2.5 mm.
 3. A composition ofmatter according to claim 1, wherein the 90% grain size of therefractory particles is not greater than 4 mm.
 4. A composition ofmatter according to claims 2 or 3, wherein the mean of the 80% and 20%grain sizes of the refractory particles is not greater than 1 mm, andthe 90% grain size of the refractory particles is not greater than 2 mm.5. A composition of matter according to any of claims 1 to 3, whereinthe mean of the 80% and 20% grain sizes of the refractory particles isat least 50 μm.
 6. A composition of matter according to any of claims 1to 3, wherein the size range spread factor of the refractory particlesis at least 1.3.
 7. A composition of matter according to any of claims 1to 3, wherein the size range spread factor of the refractory particlesis not greater than 1.9.
 8. A composition of matter according to any ofclaims 1 to 3, wherein the size range spread factor of the oxidisableparticles is not greater than 1.4.
 9. A composition of matter accordingto any of claims 1 to 3, wherein the mean of the 80% and 20% grain sizesof the oxidisable particles is not greater than 50 μm.
 10. A compositionof matter according to claim 9, wherein the 90% grain size of theoxidisable particles is not greater than 50 μm.
 11. A composition ofmatter according to claim 10, wherein the mean of the 80% and 20% grainsizes of the oxidisable particles is not greater than 15 μm.
 12. Acomposition of matter according to any of claims 1 to 3, wherein saidrefractory particles comprise one or more of sillimanite, mullite,zircon, SiO₂, ZrO₂, Al₂ O₃, MgO.
 13. A composition of matter accordingto any of claims 1 to 3, wherein at least some of the refractorymaterial has previously been fired to a temperature in excess of 0.7times its melting point expressed in °Kelvins.
 14. A composition ofmatter according to any of claims 1 to 3, wherein at least 90% by weightof any silica present in said refractory material of said mixture is inthe form of tridymite and/or cristobalite.
 15. A composition of matteraccording to any of claims 1 to 3, wherein said oxidisable particlescomprise at least one of silicon, aluminum, magnesium or zirconium. 16.A composition of matter according to any of claims 1 to 3, wherein saidoxidisable particles are present in an amount not exceeding 20% byweight of said mixture.